1. Field of the Invention
The present invention generally relates to a control apparatus for an internal combustion engine (hereinafter also referred to as the engine) of a motor vehicle for controlling optimally the air-fuel ratio of a fuel mixture supplied to the engine in dependence on engine operation state indicated by at least one sensor signal which undergoes filter processing. More particularly, the invention is concerned with an engine control apparatus capable of preventing output torque of the engine from lowering due to erroneous detection of the engine operation state which may occur upon restoration of the sensor signal to a normal level from abnormality suffering state.
2. Description of the Related Art
For a better understanding of the present invention, description will first be made in some detail of the background techniques.
FIG. 3 is a diagram showing schematically a structure of a conventional engine control apparatus together with an engine body. Referring to the figure, an internal combustion engine 1 including a plurality of cylinders (four cylinders in the case of the illustrated example) is equipped with an intake pipe 2 for supplying a fuel-containing mixture gas to the engine 1 and an exhaust pipe 3 for discharging an exhaust gas resulting from combustion of the fuel mixture within the engine cylinders.
An air flow sensor 4 is installed in the intake pipe 2 at an upstream side thereof. This sensor 4 may be constituted by a Karman vortex type sensor which generates a pulse-like signal Q indicative of an intake air flow rate. A throttle valve 5 is mounted in the pipe 2 at a position downstream of the air flow sensor 4 for controlling the intake air flow in dependence on depression of an accelerator pedal (not shown). A throttle sensor 6 is provided in association with the throttle valve 5 for detecting the opening degree thereof. Fuel injectors 7 are also mounted in an intake manifold in the vicinity of the engine cylinders for injecting fuel into the individual cylinders. A water temperature sensor 8 is provided for detecting the temperature K of a cooling water of the engine 1.
A signal representing the opening degree .phi. of the throttle valve 5 which corresponds to a torque command issued by the driver is generated by the throttle sensor 6 and used in the processing for determining the accelerating state of the engine. Further, the throttle sensor signal can be utilized for determining the fuel injection quantity in place of the intake air flow signal Q when a fault occurs in the air flow sensor 4, as will be described hereinafter, because the throttle opening degree .phi. bears a certain correlation to the intake air flow Q.
At this juncture, it should be mentioned that in addition to the above sensors 4, 6 and 8 destined for detecting the engine operation state parameters Q, .phi. and K mentioned above, there are provided other various sensors for detecting other parameters indicative of the engine operation state. Accordingly, it should be understood that the phrase "sensor means" as used herein covers the other various sensors as well although they are omitted from illustration for simplification.
A crank angle sensor 9 is provided in association with a crank shaft of the engine 1 for generating a pulselike reference period signal .theta. in synchronism with rotation (rpm) of the crank shaft and hence that of the engine 1. The reference period signal .theta. represents crank angular positions which serve as references for the various engine controls inclusive of the fuel injection control, ignition timing control and others of the engine 1. Besides, a signal Ne representing the engine speed (rpm) can be derived from the reference period signal .theta..
An air-fuel ratio controller 10 is incorporated in an engine control unit (ECU) which may be constituted by a microcomputer and serves for determining a fuel injection amount, i.e., a driving time or duration Ti of the fuel injectors 7 on the basis of the various sensor signals and the reference period signal .theta. to thereby generate a command signal J corresponding to the driving time Ti for controlling the fuel injector 7.
The air-fuel ratio controller 10 includes a decision means for making decision as to normality or abnormality of the sensor signals and a filter means for filtering at least one of the sensor signals indicative of the engine operation state (e.g. the intake air flow signal Q and/or the water temperature signal K) and serves to decide whether or not the one sensor signal (e.g. the intake air flow signal Q) is normal to thereby determine the driving duration or time Ti of the fuel injectors on the basis of other engine operation state signal (e.g. the throttle opening degree signal .phi.) when the one sensor signal is abnormal while determining the driving time Ti on the basis of the above-mentioned one sensor signal (i.e., the intake air flow signal Q) upon restoration thereof to the normal state or level.
The signals produced by the various sensors and representing the engine operation state are ordinarily subjected to the filtering or filter processing in the air-fuel ratio controller 10 for the purpose of eliminating noise components superposed on the sensor signals due to turn-on/off of various switches. In this conjunction, it is noted that the intake air flow signal Q outputted from the air flow sensor 4 mounted in the intake pipe 2 takes a relatively long time for rising up to the normal level which reflects the amount of intake air supplied actually to the engine 1. For this reason, the air intake flow signal Q undergoes the filter processing with a large time constant, whereby an intake air flow data signal A which can be used for determining the injector driving time Ti is ultimately derived. Similarly, the output signal of the water temperature sensor 8 undergoes the filter processing with a large time constant since the water temperature K is usually controlled to be constant at 80.degree. C. and less susceptible to abrupt change.
As mentioned previously, the air-fuel ratio controller 10 is incorporated in the computer-based engine control unit which is in charge of overall engine control and serves for the control of the air-fuel ratio on the basis of the sensor signal(s) and the reference period signal .theta..
FIG. 4 is a timing chart of the reference period signal .theta. and the command signal J. As can be seen in the figure, the reference period signal .theta. is of a pulse-like waveform and has a period of 180.degree. in terms of the crank angle, wherein the trailing or falling edge of the pulse is used as a reference for the fuel injection timing. In the case of the illustrated engine system, the fuel injectors 7 for all the cylinders are simultaneously driven at every crank angle of 360.degree. (upon every complete rotation of the crank shaft 1). The individual sensor signals are fetched at the falling timing t.sub.11 ; t.sub.12 of the reference period pulse signal .theta., whereby the fuel injectors 7 are actuated for the driving time or period T.sub.i (1); T.sub.i (2).
FIGS. 5 and 6 are views illustrating maps which are incorporated in the air-fuel ratio controller to be referenced in determining the amount to fuel to be injected. To this end, desired or target air-fuel ratios (e.g. 14.7, etc.) are contained in these maps as the data representing corresponding fuel amounts or quantities.
More specifically, FIG. 5 shows a fuel quantity map containing the air fuel ratios. This map is previously prepared on the basis of the engine speed Ne (rpm) derived from the reference period signal .theta. and the intake air flow data A derived from the intake air flow sensor signal Q, wherein the fuel quantities corresponding to the air-fuel ratios in two-dimensionally arrayed areas of the map are selectively determined in accordance with a function given generally in the form of f(Ne, A).
The fuel mixture having the density represented by the air-fuel ratio and injected through the injectors 7 during the driving time Ti is so controlled as to become lean as the engine speed Ne (rpm) increases and become rich as the intake air flow A increases, as can be seen from FIG. 5.
On the other hand, FIG. 6 shows a fuel map which is prepared on the basis of the engine speed Ne (rpm) and the throttle opening degree .phi. which can represent indirectly the intake air flow A. This map is used when the intake air flow signal Q suffers abnormality, wherein the fuel quantity corresponding to each of the air-fuel ratios stored in the two-dimensionally arrayed areas of the map is selectively determined in accordance with a function generally given in the form of f(Ne, .phi.).
Next, description will turn to the air-fuel ratio control operation of the conventional engine control apparatus by referring to FIGS. 3 to 6 together with a timing chart shown in FIG. 7.
Usually, the pulse-like intake air flow signal Q indicative of the intake air flow rate is outputted from the air flow sensor 4 so far as it operates normally. This intake air flow signal Q undergoes the filter processing in the fuel-air ratio controller 10, whereby the intake air flow data signal A is generated. On the basis of this intake air flow data A and the engine rotation speed Ne, a reference fuel quantity corresponding to a desired or target air-fuel ratio is determined in accordance with the function f(Ne, A) by referencing the map shown in FIG. 5. A command signal J is then generated on the basis of the reference fuel quantity by the air-fuel ratio controller 10 and applied to the fuel injectors 7. In this manner, the fuel injection can be controlled in dependence on the operation state of the engine.
More specifically, the operation state of the engine 1 is detected at a time point corresponding to the falling edge of the reference period pulse signal .theta. at every complete rotation of the crank shaft. A the same time, it is decided whether the intake air flow signal Q of the air flow sensor 4 and the water temperature signal K of the water temperature sensor 8 are normal. When these signals Q and K are normal, the injector driving time Ti is calculated in the manner described above, whereby the command pulse signal J having the pulse width or duration corresponding to the injector driving time T is generated.
At that time, the injector driving time Ti, i.e., the pulse width of the command signal J, is determined in accordance with the following expression (1): EQU Ti=f(Ne, A).times.Gi.times.Kw+Td (1)
where f(Ne, A) represents the reference fuel quantity for realizing the desired air-fuel ratio as a function of the rotation speed Ne of the engine and the intake air flow data A. Assuming, by way of example, that the engine speed Ne is 3000 rpm, the intake air flow data A is 2 g/cylinder, and that the target air-fuel ration is 15.0, the reference fuel amount of 2/15 (.times.0.133 g) is set as the map data shown in FIG. 5. Further, in the above-mentioned expression (1), Gi represents a gain for the driving time of fuel injector 7, which gain Gi is used for calculating the injector driving time Ti required for injecting the reference fuel quantity determined from the map data f(Ne, A). Furthermore, Kw represents a correcting coefficient for the water temperature signal K. This correcting coefficient Kw is set at a large value when the water temperature K is low. This is because carburetion susceptibility of the fuel is poor when the water temperature K is low, as in the case of the engine warming operation, and thus it is required to increase the fuel supply to ensure a sufficient amount of the fuel which contributes to combustion within the engine cylinders even in the engine operation state where the water temperature K is low. Finally, Td represents a dead time intervening between the reception of the command signal J and the actual start of the fuel injection. The dead time T depends on the battery voltage.
As will be apparent from the above description, the fuel injector driving time Ti is calculated in accordance with the expression (1) so long as the air flow sensor is normal.
On the other hand, when the intake air flow signal Q is at an abnormal level or unavailable from the air flow sensor 4 due to a fault (such as contact failure), the air-fuel ratio controller 10 decides abnormality of the intake air flow signal Q and calculates the injector driving time Ti in accordance with the undermentioned expression (2) by referencing the map data f(Ne, .phi.) which is prepared on the basis of the throttle opening degree .phi. as shown in FIG. 6. EQU Ti=f(Ne, .phi.).times.Gi.times.Kw+Td (2)
At this juncture, it should be mentioned that although the throttle opening degree .phi. can not represent the intake air flow rate with a high fidelity when compared with the output signal of the air flow sensor 4, the throttle opening degree can adequately be utilized as the back-up data replacing the intake air flow data A. Further, since the throttle opening degree signal .phi. is not subjected to the filter processing with a large time constant, there arises practically no problem when the input data to the air-fuel ratio controller 10 is changed over to the throttle opening degree signal .phi. from the intake air flow data A immediately upon detection of occurrence of abnormality in the latter.
In this conjunction, it is further to be noted that when the intake air flow signal Q resumes the normal level due to, for example, restoration of the contact from the disconnected state in the air flow sensor 4 at a time point t.sub.1 shown in FIG. 7, the air-fuel ratio controller 10 decides that the intake air flow signal Q is normal and calculates the fuel injector driving time Ti on the basis of the intake air flow data A in accordance with the expression (1).
However, it is at a time point t.sub.2 that the intake air flow data signal A resulting from the filter processing reaches a normal value or level A.sub.o corresponding to the actual intake air flow signal Q, as shown in FIG. 7. Consequently, during a period extending from the time point t.sub.1 to t.sub.2 after restoration of the air-flow sensor to the normal state, the driving time T.sub.1 is calculated in accordance with the expression (1) by referencing the map data shown in FIG. 5 on the basis of the intake air flow data A which assumes a lower value than the normal level A.sub.o indicating the actual intake air flow rate. As a result of this, the target air-fuel ratio is set to a value corresponding to a lean fuel gas mixture, which means that the fuel injector driving time Ti becomes shorter as shown in FIG. 7, lowering the engine rotation speed Ne and the output torque, giving rise to a problem. In this case, if the engine is in the idling state with an inherently low output torque, the engine stoppage may take place in the worst case.
On the other hand, unless the water temperature signal K is obtained from the water temperature sensor 8, the air-fuel ratio controller 10 decides abnormality of the water temperature sensor 8 and corrects the fuel injector driving time Ti in accordance with the undermentioned expression (3) by using a predetermined correction coefficient Kw(80) for a predetermined water temperature of 80.degree. C. in place of the correcting coefficient Kw corresponding to the actual water temperature K. EQU Ti.times.f(Ne, A).times.Gi.times.Kw(80)+Td (3)
In the above expression, the predetermined correcting coefficient Kw(80) is employed in place of the coefficient Kw in the expression (1). It goes however without saying that the coefficient Kw(80) may equally be used in the expression (2) as well. In this manner, when the water temperature sensor 8 is decided as suffering from a fault, the fuel injector driving time Ti is calculated in accordance with the expression (3). However, when the water temperature sensor signal is restored to the normal level, the fuel injector driving time Ti is then calculated in accordance with the expression (1) by using the correcting coefficient Kw. In that case, however, a time lag is involved in the restoration of the water temperature signal K to the normal level due to the filter processing, as mentioned hereinbefore, which results in intervention of a time lag in optimizing the fuel injector driving time Ti, to a disadvantage.
As is apparent from the foregoing description, in the case of the conventional fuel injection control system described above, the sensor signals are used as the control information by the air-fuel ratio controller 10 immediately when they are restored to the normal level, incurring a time lag in realizing the optimal driving time Ti for the fuel injectors due to incorrect detection of the engine operation state, giving rise to a problem that the engine output torque is undesirably lowered.